Metals such as aluminum can readily form a tenacious passivation layer when exposed to ambient conditions. In particular, aluminum forms a thin surface layer of aluminum oxide when exposed to oxygen from the air or water. In some applications, a layer of aluminum oxide is desirable because it can serve as a protective coating for the aluminum surface. In some applications, the natural oxide layer is increased in thickness using an anodizing process to enhance the durability and corrosion resistance of an aluminum part.
However, an aluminum oxide passivation layer can have some disadvantages. For example, the aluminum oxide layer can prevent good adhesion of a subsequently deposited metal layer. That is, the metal layer does not bond well with the aluminum oxide so the metal layer tends to peel away from or scratch away from the surface of the aluminum part. Removing this aluminum oxide layer can be difficult since the surfaces of aluminum can so readily oxidize. Even if the aluminum oxide layer is removed, a new aluminum oxide layer quickly forms back on the surface when exposed to air or an aqueous medium, such as an aqueous electrodeposition medium.